The present invention relates generally to a startup control for a chiller system. More specifically, the present invention relates to determining the number of compressors in a multiple compressor chiller system to start during the startup process for a multiple compressor chiller system.
Many liquid chiller or refrigeration applications use multiple compressors, i.e., two or more compressors, in one or more corresponding refrigerant circuits. One purpose for the use of multiple compressors is to obtain an increased capacity from the chiller system, which increased capacity could not be obtained by operating a single compressor. In addition, the use of multiple compressors can provide for improved reliability of the overall system by having one or more compressors remain operational to provide a reduced level of cooling capacity in the event that a compressor fails and can no longer provide cooling capacity.
The compressor motors of the chiller system can be powered directly from the AC power grid at the system location, which would result in the compressor being operated at only a single speed. Alternatively, the compressor motors can use a variable speed drive inserted between the system power grid and the motor to provide the motor with power at a variable frequency and variable voltage, which then results in the compressor being capable of operation at several different speeds. Variable speed operation of the motors can be obtained by providing a corresponding variable speed drive for each compressor motor or by connecting all of the compressor motors in parallel to the inverter output of a variable speed drive. One drawback of using a variable speed drive for each compressor is that the overall chiller system becomes more expensive because multiple drives with a given cumulative power rating are more expensive than a single drive of the same output power rating. One drawback to connecting the compressor motors in parallel to the single inverter output of the variable speed drive is that a fault or failure of one of the motors may disable the variable speed drive and thus prevent the other motors connected to the variable speed drive from operating the remaining compressors on the chiller system. This disabling of the other motors connected to the variable speed drive defeats the function of the redundant compressors because all the compressors are disabled as a result of the disabling of the motors and the variable speed drive.
One startup control for a multiple compressor system, whether the compressor motors are powered by the AC power grid or by variable speed drives, involves the starting of a lead compressor followed by the subsequent starting of additional compressors. One example of this type of control can be found in U.S. Pat. No. 4,614,089 (the '089 patent). The '089 patent is directed to controlling the operation of refrigeration systems which contain multiple compressors. The control has a “power-up set” function that activates a delay which can be set individually, and thus differently, for each of the compressors. Upon the system startup, one compressor can be started, for example, every 30 seconds until all compressors, or fewer if the desired suction pressure is achieved, are back on line. One drawback to this technique is that the maximum number of compressor cannot be started at one time and can only be obtained after a delayed time period.
Therefore, what is needed is a system and method for starting a multiple compressor chiller system that can determine the maximum number of compressors to start for a given system load.